In aircraft, it is usual to provide oxygen supply devices with which the cockpit crew can be supplied with the requisite oxygen, for example in the event of decompression of the cockpit. These oxygen supply devices typically have an oxygen mask connected by a line to an oxygen reservoir. In the event of faulty behaviour or failure of these oxygen supply devices, replacement devices are normally also carried in aircraft, the use of which ensures the supply of oxygen to the pilot even in the event of failure of the actual oxygen supply device.
In the past, however, it has often been shown that such a failure of the oxygen supply device was not even noticed, which led to the pilots not using the replacement device also carried, despite there being a deficiency of oxygen. At high flight altitudes, this normally leads to a loss of consciousness of the pilot within a short time and, associated with this, often to the aircraft crashing.